The present invention relates to a crosslinkable copolymer composition or, more particularly, to a copolymer composition comprising a vinylic, acrylic, styrenic or other organic copolymer and, for example, an organic peroxide as a crosslinking or vulcanizing agent and a method for the preparation thereof.
It is known in the prior art that an organic polymer can be rendered crosslinkable or vulcanizable by introducing organosilicon moieties having a silicon-bonded hydrolyzable group as the monomeric units into the polymeric molecular chain so that exposure of the thus obtained copolymer to atmospheric moisture causes hydrolysis of the silicon-bonded hydrolyzable groups into silanols followed by the silanol condensation therebetween to form intermolecular crosslinks.
For example, U.S. Pat. No. 3,632,557 teaches a urethane polymer terminated at the molecular chain ends with alkoxysilyl groups of the formula Si(OR).sub.n, R being a monovalent hydrocarbon group and n being an integer of 1, 2 or 3; U.S. Pat. No. 3,503,943 teaches an elastomeric copolymer of ethylene, propylene and a dienic monomer having SiR.sub.2 X groups, R being as defined above and X being a hydrolyzable group, introduced thereinto by the grafting reaction of hydrosilation with a silane of the formula HSiR.sub.2 X; Japanese Patent Publication No. 48-1711 teaches an olefinic (co)polymer to which organosilicon moieties having at least two hydrolyzable groups are introduced by the grafting reaction of an organosilane compound having an olefinically unsaturated group by the aid of a free-radical generating agent; U.S. Pat. No. 3,471,440 teaches a dienic polymer having been subjected to an addition reaction with an organosilane compound having a hydrolyzable group; and U.S. Pat. No. 3,971,751 teaches a polyether compound having been subjected to an addition reaction with an organosilane compound having a hydrolyzable group.
A problem in the crosslinkable organic polymers prepared by the above described prior art methods in general is the relatively low velocity of the crosslinking reaction taking an unduly long time for complete curing of the polymer although the crosslinking reaction can proceed even at room temperature by the exposure to the atmospheric moisture as is mentioned above. When the base polymer has a low permeability to moisture, this problem is more serious and the core portion of a thick body of the polymeric composition can hardly be cured to completeness. Moreover, the process of crosslinking by the hydrolysis and silanol condensation reactions necessarily produces volatile byproducts which may adversely affect the properties of the products obtained by the crosslinking or vulcanization.
As is known hitherto, in particular, acrylic rubbers have excellent resistance against heat, oils and ozone in comparison with other synthetic rubbers so that they are widely used in various fields of applications where heat resistance, aging resistance and oil resistance are essential such as sealing materials, gaskets, rubber hoses in chemical plants and the like. The acrylic rubbers currently produced and used industrially include several different types of which the principal monomeric constituent is an acrylic acid ester copolymerized with a relatively small amount of an active chlorine-containing monomer such as 2-chlorovinyl ethers, vinyl monochloroacetate and the like or an epoxy-containing monomer such as glycidyl methacrylate, allyl glycidyl ether and the like.
These prior art acrylic rubbers, however, have several disadvantages and problems that the velocity of vulcanization thereof is considerably smaller than other general-purpose synthetic rubbers requiring a procedure of vulcanization in which the rubber polymer is admixed with a polyamine-based or a sulfur/soap-based vulcanizing agent and heated in press at a temperature of about 170.degree. to 180.degree. C. for several tens of minutes followed by post-vulcanization for several hours, that the rubber compounds formulated with a copolymer of an active chlorine-containing monomer are liable to cause corrosion of metal molds used in the fabrication of rubber articles and that the use of a polyamine-based vulcanizing agent is objectionable when the rubber article is to be used in contact with medicines and foods due to their toxicity.
Various proposals and attempts have been made with an object to overcome the above described disadvantages and problems by introducing unsaturated linkages into the pendant groups on the molecular chain of the rubbery polymer so as to improve the crosslinking behavior, corrosiveness or toxicity of the rubber. For example, an acrylic acid ester is copolymerized with a dienic hydrocarbon compound (see U.S. Pat. No. 2,643,247), derivative of cyclopentadiene (see U.S. Pat. Nos. 3,402,158 and 3,487,057) or cyclohexene (see U.S. Pat. No. 3,497,571) and the like so that the resultant rubbery polymer can be vulcanized with sulfur or an organic peroxide as the vulcanizing agent. The improvements obtained by these modifications are still quite insufficient and impracticable with rather decreased velocity of vulcanization or very poor workability of the rubber compound in processing.
A recently proposed improvement concerns the use of a copolymer of an acrylic ester and a norbornene derivative as the acrylic rubber base which is susceptible to the vulcanization with sulfur or an organic peroxide as the vulcanizing agent. Although the improvement is considerable in respect of the vulcanizability with sulfur, the velocity of vulcanization is still quite unsatisfactory when the vulcanizing agent is an organic peroxide so that this method is also industrially unacceptable.